Modern projection systems display video and digital information for business, commercial, technical and residential use. One form of display system can be a plasma display, which generates its own light.
Projection systems require a light source that is both compact and high in brightness. Many projection systems require a separate source of bright light, and frequently employ high intensity discharge (HID) lamps, many of which use electrodes and metal salt additives to improve their light quality. Electroded HID lamps suffer from a problem known as "meltback", and deteriorate over time.
The throughput or screen illuminance of an optical system is highly dependent on the compactness of the light source, its luminance (lumens per steradian-mm.sup.2 or candela/mm.sup.2), or brilliance.
As aforementioned, a number of HID lamps having electrodes are currently used in projection display systems. A drawback of these electrode HID lamps is that they are prone to electrode meltback due to the high power and aggressive chemistry used to generate appropriate colors. The advantage of these lamps, however, is high lumen output, high luminance, good color, and small arc gap. A small arc gap is essential for coupling the light through the optical system. Luminances in these lamps approach 500 cd/mm.sup.2.
Examples of electrode HID lamps are the OSRAM Model No. HTI 150 W. and the Model No. HTI 250 W/22,32, manufactured by the assignee. The highest luminance point of these HID lamps is at their ends in front of the electrodes. Consequently, there are two hot spots. The projection or optical system can accommodate only one luminance point, and therefore a portion of the light must be discarded. As the electrodes melt, or burn back, the hot spot is moved from the optical focus, thus causing throughput deterioration.
The present invention is a new electrodeless high intensity discharge (EHID) lamp for photo optical applications. The new EHID lamp has a unique construction that provides high luminance.
The current invention reflects the discovery that constricting the mid-portion of the lamp capsule will yield a higher luminance output.
The invention also features an improved cooling arrangement for the lamp capsule, which provides longer operative life.
The present invention has its highest luminance point away from the ends of the capsule (i.e., in the center of the tube). This center luminance stays in the same place over time.